The objective of the proposed research is to further investigate axon growth in development. In particular, it is proposed to extend two recent discoveries on this theme. The first is the finding that impulse activity in the retina and optic nerve plays little or no role in the development of retino-tectal connections. Experiments are proposed which ask whether impulse activity is required in tectal cells for the proper development of this connection. These experiments will be done by the embryonic transplantation of the dorsal mesencephalon (the tectal primordium) from one species of salamander whose neurons are sensitive tetrodotoxin (which blocks impulse activity) to another insensitive species which manufactures and contains this toxin. In theoretically similar experiments both the retinal and tectal elements will be deprived of impulse activity throughout development. The other major goal of the proposed studies is to further examine the discovery that the embryonic salamander brain is divided into a number of discrete and internally homogeneous zones or territories which influence the trajectories of transplanted optic tracts. The questions to be asked are: 1) Do these territories influence the trajectories of other nerve tracts? To answer this, olfactory placodes will be transplanted and the associated olfactory nerves will be followed using neuroanatomical tracing methods. 2) Are the boundaries between territories shifted by a genetic deformation in the developing brain which leads to eyelessness? To answer this question embryonic optic vesicles will be transplanted from normal animals to mutants with this defect.